This invention relates to amplitude modulation (AM) stereo receivers, and particularly to AM stero receivers which are capable of receiving broadcast stereo signals having composite amplitude and angular modulation impressed on a carrier according to different composite modulation standards.
At least five different approaches have been proposed for the implementation of stereophonic broadcasting in connection with the existing AM radio service. See, for example, the article entitled "AM Stereo: Five Competing Options" published in the IEEE "Spectrum" magazine of June 1978, at page 24, and the public file of the Federal Communications Commission's (FCC's) Docket No. 21313, the AM Stereo Broadcasting proceeding. Each of the five systems described therein uses a different modulation technique for providing an add-on stero capability to AM transmitters and suitably equipped receivers. All five proposed systems provide a composite transmitted signal which has a compatible signal format so that existing monophonic AM receivers can detect a monophonic audio signal component from the composite signal which is transmitted in each of the systems. In addition to the monophonic signal component, receivers which are specially equipped for any one of the proposed composite modulation standards will receive a stereophonic signal component, which differentiates left (L) and right (R) audio information and can be decoded and combined with the detected monophonic signal component in order to provide stereophonic sound.
One of the proposed AM stereo systems utilizes amplitude and frequency modulation (AM/FM) to develop a composite signal for transmission. In accordance with this proposed system, a carrier is frequency modulated with information corresponding to the difference between left and right stereo audio signals (L-R). The frequency-modulated carrier is then amplitude modulated with a signal corresponding to the sum of the left and right stereo audio signals (L+R), which is equivalent to standard monophonic amplitude modulation (AM), and the resulting composite signal is broadcast. As a result, a conventional AM receiver, which utilizes an envelope detector, detects the AM or (L+R) component of the composite signal and provides monophonic reception. A specially equipped stereo receiver will also detect the frequency modulation or (L-R) component of the composite signal. The resulting (L-R) representative audio signal can be combined with the (L+R) sgnal in an additive and subtractive matrix to produce separate (L) and (R) output audio signals for stereo listening.
Another of the proposed systems utilizes phase modulation instead of frequency modulation of the carrier (AM/PM) to transmit stereo difference (L-R) information. In this system the phase-modulated carrier is then amplitude modulated with (L+R) information to develop a composite signal which is then transmitted.
Yet another of the proposed systems utilizes a modulation technique known as compatible quadrature amplitude modulation (CQUAM) to provide a modified phase modulation of a carrier with (L-R) information. The phase-modulated carrier is thereafter amplitude modulated with (L+R) information to develop a composite signal. This composite signal may also be viewed as consisting of a pair of carriers at the same frequency but separated in phase by 90 degrees (quadrature carriers), where one carrier is amplitude modulated with left (L) stereo audio information and the other with right (R) stereo information.
Still another of the proposed systems is known as the variable compatible phase multiplex (V-CPM) system and is a modified form of quadrature system. In this system two carriers at the same frequency are separated in phase by an amount which varies between 30 degrees and 90 degrees depending on the content of the audio signals being transmitted. One of these carriers is amplitude modulated with left (L) stereo audio information and the other with right (R) stereo information and the two are linearly combined. The resultant signal can be resolved into an in-phase component representative of (L+R) information and a quadrature-phase component representative of (L-R) information. (L-R) information below 200 Hz. is eliminated to provide room for a frequency-modulated, low frequency (55 to 96 Hz) pilot signal which performs two functions. It indicates the presence of a stereo broadcast, and its modulation communicates to specially equipped stereo receivers the instantaneous phase angle between the two variable-angle carriers in this system so that such receivers can track the resulting variation in phase modulation in the transmitted signal. In a corresponding stereo receiver the composite signal may be envelope detected to provide an (L+R) audio signal and quadrature synchronous detected to derive a signal which represents the (L-R) audio information. The pilot signal is separately detected and its modulation can be used to vary the gain of the (L-R) signal channel to provide the equivalent of a variable-angle receiver which tracks the broadcast signal. The resulting (L+R) and gain-controlled (L-R) signals are then combined in a conventional stereo matrix to develop (L) and (R) signals. In addition, the developer of this system has proposed a simplified receiver in which the gain of the (L-R) channel is not varied. This corresponds to receiving the variable-angle broadcast signal at a compromise fixed angle, instead of tracking the angle variation.
Finally, there is a proposed system known as the independent sideband (ISB) system. This system phase modulates the carrier with a suitably modified (L-R) signal and then amplitude modulates the phase-modulated carrier with an (L+R) signal, where the (L+R) and (L-R) signals have been phase shifted so as to be in a quadrature relationship. As a result, the lower sidebands of the resulting composite signal contain primarily left (L) stereo information whereas the upper sidebands contain primarily right (R) stereo information (hence the name "ISB"). This system is also described in the inventor's U.S. Pat. Nos. 3,218,393, 3,908,090 and 4,018,994.
The composite signal transmitted by each of the proposed systems includes a low-frequency pilot signal component for identifying the presence of a stereo broadcast. Because the pilot signal frequencies are different for each of the above-mentioned systems (AM/FM-20Hz; AM/PM-5Hz; CQUAM-25Hz; V-CPM-55 to 96 Hz; and ISB-15Hz) they also inherently identify the modulation approach used in each composite signal.
More detailed descriptions of these systems appear in the aforementioned IEEE Spectrum article, in the public file of FCC Docket 21313, and in various patents which have been issued to the proponents of these systems.
Despite significant differences in performance of the various proposed systems, the FCC has had difficulty in its attempt to choose one of these systems as the basis for a national standard for AM stereo broadcasting. As a result, the FCC is considering authorizing more than one of these systems. In this case the normal forces of free competition in the marketplace will be allowed to determine whether one of the systems will eventually become the predominant AM stereo system, or whether two or more systems can coexist.
It is, therefore, an object of the present invention to provide a receiver capable of receiving AM stereo signals which have composite modulation according to any one of two or more of the various proposed modulation techniques.
It is a further object of the present invention to provide an AM stereo receiver capable of detecting the pilot signal used in conjunction with any one of the various proposed AM stereo broadcast techniques.
It is a further object of the present invention to provide an AM stereo receiver capable of automatically distinguishing, by reason of the pilot signals, which of the various proposed modulation techniques is being used in a particular received AM stereo broadcast signal.